Question

Metamerism is the characteristic of
A. Ascaris
B. Pheretima
C. Periplaneta
D. Hydra

Answer 1

Hint: Metamerism is a phenomenon in which the body is segmented both externally and internally. Each segment thus formed is called a metamere and is arranged in a linear series. This interesting process of metameric segmentation is exhibited by organisms of certain phyla in Kingdom Animalia.

Complete answer:
Metamerism is demonstrated by organisms of phylum Annelida, Arthropoda and Chordata. Their architectural body plan exhibits serial repetition of similar body segments and organ systems.
Metamerism involves the segmentation of ectoderm and mesoderm. Externally two metamers are separated by a deep groove and internally by septa. Structurally each metamere follows the same fundamental plan and contains a part of all organ systems. Each metamere contains segmental appendages, muscles, segmental nerves and ganglion, segmental blood vessels or segmental excretory organs. Each segmental structure within a metamere is interdependent on one another. They integrate to form a single functional unit and thus, work together in coordination.

Metamerism has been classified into various types:
1. Complete Metamerism: When segmentation is observed in all organ systems, it is called complete metamerism. E.g. Earthworm, Leech, etc.
2. Incomplete Metamerism: When the segmentation is not observed in all the organ systems, it is called incomplete metamerism. E.g. Cockroach, Prawn, etc.
3. Homonymous Metamerism: In this condition, all the metameres are similar both internally and externally. E.g. Earthworm, Nereis, Leech, etc.
4. Heteronomous Metamerism: In this condition, the segments in different body regions appear dissimilar and are restricted only to certain organs. E.g. Cockroach, Prawn, etc.

Pheretima (earthworms) is a genus of Phylum Annelida. It is the best model organism for the study of metameric segmentation in organisms. It demonstrates complete and homonymous metamerism showing both external and internal body segmentation.

Hence, the correct answer is option (B).

Note: Metamerism is observed throughout the body of an organism except the head in anterior region and pygidium or telson in the posterior region. This restricts metameric segmentation to only the trunk region. Pseudo-metamerism occurs when the segmentation of the ectoderm results in the formation of segments. It is observed in few members of Phylum Platyhelminthes like tapeworms.